1. Field of the Invention
The present invention relates generally to the field of determining particle size distribution and more specifically to an optical detector used for the measurement of particle size distribution in an oscillating flow field.
2. Discussion of the Related Art
The measurement of particle size distribution finds use in the process industries in the manufacture of pharmaceuticals, chemicals, abrasives, ceramics, pigments and the like, where the particle size affects the quality of the manufactured product.
A number of methods presently exist for determining the size distribution of particulate material for particles in the approximate size range of 0.1 to 1000 microns in diameter. The conventional method of measurement at high concentration is dynamic light scattering, as taught by U.S. Pat. No. 5,094,532 to Trainer et al, patented Mar. 10, 1992. This patent discloses a fiber optic Doppler anemometer and method that directs a beam of light into a scattering medium that contains particles in Brownian motion. The frequency of the scattered light is compared to non-scattered light emitted from the scattering medium and results in the generation of a first signal having a magnitude that is indicative of the difference in frequency between the scattered light and the non-scattered light. A second signal is generated having a magnitude that varies with frequency on a linear scale. The frequency scale of the second signal is then translated into a logarithmic scale and deconvolved to determine the size and distribution of moving particles within the scattering medium. The translation and deconvolving requires translation of analog signals to digital signals and subsequent processing by a central processor and a vector signal processor using fast Fourier transfer techniques (FFT). In order to solve for a known particle size distribution of over 80 particle diameters the method just described must sample over 80 frequencies. Even though this method provides an accurate measurement of particle size distribution, it does require a long time period (usually greater than two minutes) to process all of the sample frequencies, due primarily to the stochastic nature of Brownian motion. This technique is best suited for use in a laboratory with samples that have been extracted from a process and properly prepared for measurement analysis. Additionally, this method is strongly dependent upon dispersant viscosity and temperature and the use of non-flowing sample delivery systems. Although this technique provides accurate results for particles having diameters less than 1 micron, it exhibits poor size and volume accuracy for particles greater than 1 micron.
Another recognized technique and method for measuring the size distribution of very small particles is static light scattering, or angular light scattering. In this method, a collimated monochromatic light beam irradiates an ensemble of particles that flow perpendicularly through the collimated light beam. Light scattered from the particles emerges from the interaction over a range of angles from the axis of the collimated beam. The scattered light is collected by a lens placed in the path of the scattered light. The scattered light patterns focused in the focal plane of the lens are typically measured by an array of photodetectors placed in the focal plane. The angular extent of the scatter pattern is determined by the size of the particles. The smaller the particle, the wider the angular extent of the scatter; the larger the particle, the narrower the angular extent of the scatter.
One such method is taught by U.S. Pat. No. 5,416,580 to Trainer, patented on May 16, 1998, which uses multiple light beams to irradiate the particles. This method has demonstrated excellent measurement results for particles in the 0.1 to 3000 micron range in flowing sample systems, without temperature or viscosity dependency. Unlike the dynamic scattering techniques, measurements can be made in less than five seconds with repeatability superior to that of the dynamic light scattering. However, in order to produce good measurement accuracy for a process sample at a high concentration, for example 10% by volume, the process sample must be properly diluted with a dispersant medium to minimize the particle concentration.
Each of the above described techniques is limited to a certain range of particle size, concentration and shape. Particles of many shapes are encountered in the aforementioned industrial processes. In certain applications hydrodynamic particle size measurement techniques present a better correlation to the product quality than the optical particle size measuring techniques for irregularly shaped particles. A particularly difficult region is between 0.5 and 1 microns, where both static and dynamic scattering can present somewhat of a less than accurate measurement of particle size distribution. Hydrodynamic particle size measurement techniques employ a basic concept of detecting a particle's motion, or oscillations, in a fluid dispersant caused by a vibrating surface or an ultrasonic wave. Depending on the oscillating frequency applied to the dispersant fluid, the particles will closely follow the oscillation of the dispersant fluid. The present invention contemplates the use of optical light scattering techniques for measuring the Doppler shifted light that is scattered by particles suspended in a dispersant medium and which are excited by an ultrasonic wave. The motion of the particles and dispersing fluid are compared as a function of excitation frequency to provide a determination of particle size distribution.